A cross section of a conventional permanent magnet DC motor is shown in FIG. 1. The motor 10 comprises a stator and a rotor 15 rotatably mounted confronting the stator. The stator comprises a cylindrical housing 11 with permanent magnets 14 which are fixed to the inner surface of the cylindrical housing 11 and distributed around the rotor 15. As is known, a cylindrical housing is vulnerable to rolling during transportation and is inconvenient to install.
A square motor developed in recent years is shown in FIG. 2. The square motor 20 comprises a stator and a rotor 25. The stator comprises a square housing 21 with permanent magnets 24 fixed to the inner surface of the square housing 21. The rotor 25 is rotatably mounted confronting the stator and surrounded by the magnets 24. The square motor 20 solves the above-mentioned shortcomings of the conventional cylindrical motor 10. However, the square motor 20 has a low space utilization rate since the space near the corner portions of the housing 21 is not used. Furthermore, the outer diameter of the rotor 25 is restricted by the size of the square housing 21 and the thickness of the magnets 24, especially in small motor.
Therefore, there is a desire for an improved motor having a high space utilization rate.